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Lathe speed vs. resistance to bowl flexing during the cut

E

Edward Wargo

Joined
Mar 21, 2018
Messages
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Location
Burlington, NJ
As previously mentioned, none of the above is possible.....if the tool isn't sharp, or presentation isn't at the best possible advantage. I'll add that the rpm is a critical element, as well. The best rpm will allow the most resistance to a bowl flexing during the cut, and it should be obvious to everyone that any felt vibration at any given speed is detrimental to the objective.

The quote above was posted by odie in 2006 and was part of a discussion about sharpening. This is a link to that forum. http://www.aawforum.org/community/index.php?threads/tools-arriving.14694/#post-148397

I'm relatively new to turning and am curious as to how to recognize the appropriate speed to use for a particular cut or series of cuts.
 
Lathe speed affects the feed rate of the tool and the amount of vibration in an out of balance piece.

I begin the process by mounting the piece on the lathe so that it is most balanced for weight. Do some roughing then align it to balance the grain.

A faster speed allows a more fluid feed rate and usually results in a cleaner cut.
A faster speed beyond a certain point increases vibration making a clean cut difficult to achieve.

I dial in the speed by increasing it until I feel vibration then I will increase it some more to see if the vibration smooths out at a higher speed. It usually does. If not I will slow down to where’s the vibration started.
Initial roughing I can accept more vibration as the roughing will bring the piece into weight balance reducing any vibration as I remove wood.

When the bevel drags on the wood it creates vibration. Using light bevel pressure and micro bevels reduce bevel drag. Resin from green wood building up on the bevel increases bevel drag. Sharpening the tools cleans the bevel.
 
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The other consideration is the size of the wood blank being cut, a larger out of balance piece is somewhat limited in speed by the size and mass of the lathe and the apparatus it is mounted to. A smaller wood blank can be out of balance but easily turned at a higher speed to quickly rough the blank to round, compared to a larger piece which usually is turned at a slower speed and creates difficulties during the time it takes too bring the piece into round as it can be a physical contest between the out of balance wood blank and the wood turner as to which one wins.
 
Al pretty much nailed it. I turn a lot of relatively small turnings and higher speed lets me move my body in a more comfortable pace. Since I control the gouge with my body it lets me get a smoother arc on my turnings. Faster speed usually means the gouge is taking a smaller bite per revolution so you get a cleaner cut most of the time. The problems with promoting faster speeds however is the new turner often has wood that is suspect due to poor harvesting and has cracks. Then they are also turning larger bowls. Large bowls, cracks, and speed are not a good thing. The people selling the scraping carbides promote faster speeds to get cleaner cuts. I think this combination of faster speeds, poor tool handling with scrapers, and inexperience with the wood is just an accident waiting to happen. Now I'm not dissing carbide scrapers. Used properly they work really well, it's just the combination of all those elements that gets dangerous. So I'm always a little hesitant to recommend speed as a solution to better cuts.
 
The quote talks a\bout flexing in the bowl but so far the comments talked about out of balance vibrations, which seams to be a different problem. The bowl walls will flex when the thickness is smaller relative to the size of the bowl and farther from the center. The way that I solve the problem is to first finish one side usually the outside, then begin to finish the other side in stages such that overall finish cuts are unnecessary. The other problem this solves is the tendency for the wood to change shape as stress are relieved making it impossible to to go back.
 
A rotating machine and billet produce various resonant frequencies and vibrations depending on the RPM's and the changing mass and balance of the piece. You can dampen the resonance by changing the speed of the lathe or adding or removing weight to the billet. You could also apply elastic bands or straps around the exterior of the billet to dampen flexing or inflate a hollow ball, bag, innertube, etc. inside the vessel to dampen the flexing. You could also soak the wood in water and freeze it to allow turning the piece, or you could use resin to stabilize the piece which would dampen the flexing. There is no one solution for this phenomena, there are multiple variables that need to be considered which are constantly changing with the piece being turned.
 
Well, wood bowl shapes will flex. How much depends on several things. One is thickness. Another is size/diameter. Another is speed/revs/rpm. Another is bevel pressure. Oh, I do mostly green once turned bowls, so my views are skewed to that perspective.

The thinner the bowl shape is the more likely it is to flex as you turn out the inside. It will elongate along grain likes, longer in the up/down grain lines, less in the side grain, and with really wet once turned pieces, losing water can contribute to this as well. A one inch thick bowl will not flex as much as a 1/4 inch thick bowl.

Size matters. On smaller pieces, say 10 inches or so and down, it isn't too difficult to make one final cut from rim to base if you have the proper gouge nose angle, usually in the 60 degree range. The bowl may move a little as you remove bulk, but that is minimal. On deeper and bigger bowls, some where in the 4 or so inch deep and 12 or so inch diameter, Things begins to change. You end up having to turn out the inside in stages. You do down an inch or two, do what ever clean up is necessary (shear scrape/NRS/another very light cut) and then go down another inch or two, gently picking up the cut from where you left off. There is generally a little line at the pick up point because the bowl has moved.

The higher the speeds are, the more likely you are go get distortion in the bowl shape as it spins. If you are turning end grain, this is almost non existent. With side grain, due to issues of end grain/side grain, the piece will not spin true. Diameter and rpm can make pretty big differences.

An old quote I love to use, and have no idea who said it first, "The bevel should rub the wood, but the wood shouldn't know it." On the outside of the bowl, you can push the bevel into the wood fairly hard, but since there is a lot of mass on the other side, the wood doesn't move, but you do get burnishing of the wood. There is always a tendency to push on or into the bevel rather than pushing into the cut. On the inside of the bowl, this can make a huge difference. You have no mass pushing back to you, so if you are pushing into the bevel rather than into the cut, you will get a lot of movement. In this case, wall thickness/thinness as well as grain orientation will cause the bowl to flex. Probably the most difficult thing to learn is to float the bevel as Stuart Batty says. The whole thing about turning in general is that it doesn't take brute strength to turn. If you are having to use that, then your tools are most likely dull.

Perhaps one final point, I think green wood flexes a lot more than dry wood. This also could be partly due to moisture loss during turning.

robo hippy
 
robo hippy said:
Well, wood bowl shapes will flex. How much depends on several things. One is thickness. Another is size/diameter. Another is speed/revs/rpm. Another is bevel pressure. Oh, I do mostly green once turned bowls, so my views are skewed to that perspective.

The thinner the bowl shape is the more likely it is to flex as you turn out the inside. It will elongate along grain likes, longer in the up/down grain lines, less in the side grain, and with really wet once turned pieces, losing water can contribute to this as well. A one inch thick bowl will not flex as much as a 1/4 inch thick bowl.

Size matters. On smaller pieces, say 10 inches or so and down, it isn't too difficult to make one final cut from rim to base if you have the proper gouge nose angle, usually in the 60 degree range. The bowl may move a little as you remove bulk, but that is minimal. On deeper and bigger bowls, some where in the 4 or so inch deep and 12 or so inch diameter, Things begins to change. You end up having to turn out the inside in stages. You do down an inch or two, do what ever clean up is necessary (shear scrape/NRS/another very light cut) and then go down another inch or two, gently picking up the cut from where you left off. There is generally a little line at the pick up point because the bowl has moved.

The higher the speeds are, the more likely you are go get distortion in the bowl shape as it spins. If you are turning end grain, this is almost non existent. With side grain, due to issues of end grain/side grain, the piece will not spin true. Diameter and rpm can make pretty big differences.

An old quote I love to use, and have no idea who said it first, "The bevel should rub the wood, but the wood shouldn't know it." On the outside of the bowl, you can push the bevel into the wood fairly hard, but since there is a lot of mass on the other side, the wood doesn't move, but you do get burnishing of the wood. There is always a tendency to push on or into the bevel rather than pushing into the cut. On the inside of the bowl, this can make a huge difference. You have no mass pushing back to you, so if you are pushing into the bevel rather than into the cut, you will get a lot of movement. In this case, wall thickness/thinness as well as grain orientation will cause the bowl to flex. Probably the most difficult thing to learn is to float the bevel as Stuart Batty says. The whole thing about turning in general is that it doesn't take brute strength to turn. If you are having to use that, then your tools are most likely dull.

Perhaps one final point, I think green wood flexes a lot more than dry wood. This also could be partly due to moisture loss during turning.

robo hippy
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In regards to the line between stages of cut as you work your way through the stages the final negative rake shear scrap should remove the mark between the stages. I have a scraper the I developed on a 3/4" bar that seams to work to introduce the bur at the necessary angle and will try for a photo of it in action.
 
DSC00195.JPGThe tool shown has a bur formed to do the cutting and obviously needs to be presented better than this still photo shows. The end of the rod is machined to a 45 degree angle and the rod can be held with an arm brace handle or used by itself. The 3/4" bar may be a little overkill but it does work well for a controlled light touch that is required.
 
Some times the NRS works, though that depends on the wood. I generally stay away from the NRS on bowl walls as it is still a scraping cut and will pull more on the fiber. Some times the shear scrape works, and I prefer that to the NRS, most of the time. A NRS can do a good shear scrape if you have a burnished burr on it which is very long lasting.

robo hippy
 
robo hippy said:
Some times the NRS works, though that depends on the wood. I generally stay away from the NRS on bowl walls as it is still a scraping cut and will pull more on the fiber. Some times the shear scrape works, and I prefer that to the NRS, most of the time. A NRS can do a good shear scrape if you have a burnished burr on it which is very long lasting.

robo hippy
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If by NRS you mean a double ground burred scarper sitting flat on the tool rest then yes it would rarely work on a side grain bowl. The original term "shear scarping" as I learned it in 1990 applied to a burnished bur presented at negative rake shear angle. The term "negative rake" was not bothered with because it obvious that the bur is not going to cut any other way.
 
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We have different ideas of 'shear scrape'. Negative rake was coined by Stuart Batty, comparing the cut it does to some cross cutting saw blades, and some made for laminates where the teeth are angled back away from the cut. This prevents any lifting of the piece being cut. Shear scrape, to me is a slicing cut, and called a scrape because it the bevel is not rubbing. It is not a scrape in any way, shape, or form to me, and I don't think any one uses scrapers as much as I do. Most do use gouges for their shear scraping cuts, and the swept back grinds are preferred for this. I use several specialized scrapers for all of my shear scraping. I have them up on edge at a 45 to 70 degree angle, depending on the wood. I have no clue as to how the 'negative rake' term applies to shear scraping.

robo hippy
 
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